1. Field of the Invention
The invention relates generally to semiconducting devices and, more specifically, to PIN photodetector diodes.
2. Description of Related Art
Existing PIN diodes include an absorption region in which incident illumination generates electron-hole charge carriers, and a depletion region in which the density of charge carriers is insufficient to neutralize the fixed charge density of donors and acceptors. Light incident upon the depletion region causes a change in PIN photodiode conductivity. In the context of optical communications, this conductivity change has been advantageously exploited to detect multichannel signals carried on fiber optic cables. Unfortunately, however, presently-existing PIN photodiodes exhibit insufficient linearity when conductivity is measured as a function of illumination. This shortcoming is apparent when the photodiode is employed in the context of cable television to process a plurality of frequency-division-multiplexed, amplitude-modulated vestigial-sideband (AM-VSB) video signals. Nonlinearities in photodiode conductivity as a function of illumination distort the envelope of VSB video signals, providing a degraded video signal at the customer premises.
The invention is based upon our discovery that presently-existing PIN diodes are nonlinear because some of the charge carriers generated by illumination do not change the conductivity of the diode. In particular, charge carriers generated outside of the depletion region become trapped by semiconductor donors and acceptors, and are thus unable to enhance photodiode conductivity. Due to the fact that existing PIN diodes absorb some light in areas outside of the depletion region, and this light absorption does not lead to a corresponding change in conductivity, the photodiode exhibits nonlinearities. A further complication is that the boundaries of the depletion region change dynamically during photodiode operation, thereby rendering linearity all the more difficult to achieve.
In view of the foregoing, an improved PIN photodiode embodying the principles of the invention provides enhanced linearity by confining the light absorption region of the diode wholly within the diode""s charge carrier depletion region. The photodiode exhibits improved linearity over prior art designs because the thickness of the absorption region is no longer a function of changes in the size of the depletion region during device operation. Keeping the absorption region wholly within the depletion region ensures that a greater number of the charge carriers generated by incident illumination will increase the conductivity of the semiconductor material relative to prior art designs.